On Wednesday, French authorities revealed the likely cause of the 2016 crash of EgyptAir flight MS804. The flight, which suddenly went off the radar almost six years ago, on May 19, was traveling from Paris to Cairo when it crashed into the Mediterranean Sea, killing all 66 people on board; 56 passengers and ten flight staff.

According to CBS News, experts now say the crash was caused by a fire originating in the cockpit, which was likely sparked by “a leak from the co-pilot’s oxygen mask and the combustion of a cigarette smoked by the pilot or co-pilot.”

Initially, Egypt‘s aviation minister attributed the crash to a terrorist attack. However, automated messages sent by the airplane indicated that prior to the aircraft losing contact, there had been smoke in the cabin and an issue with the flight control unit.

A U.S. source interviewed by CBS News in 2016 also stated there were indications of damage caused by elevated temperatures as well as evidence of the presence of smoke.

When the cockpit voice recorder and flight data recorder for the EgyptAir flight were recovered from the sea a day apart, the recordings also helped rule out the terrorism theory. What was described as rustling sounds were heard from the microphone inside the co-pilot’s mask, likely strong air flow indicating that the mask’s emergency mode was activated. The fire itself was determined to have started by a spark or flame which was most likely caused by a cigarette.

In 2018, experts noted the replacement of the co-pilot’s oxygen mask, which they said “requires very careful verification” as oxygen leaks are “particularly dangerous.” The same year, The French Civil Aviation Safety Investigation Bureau released a report in which they stated it was their belief that a fire had started in the cockpit.

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